1. Fields of the Invention
This invention is directed to a method of and apparatus for identifying presensitized offset printing plates having thereon photosensitive layers of different light reflective characteristics, more particularly to a method of and apparatus for identifying the printing plate by examining the light reflectances from the photosensitive layer thereon in four colors.
2. Description of the Prior Art
There have been provided on the market various kinds of presensitized offset printing plates which require different developing procedures. These presensitized plates, generally composed of an aluminium-made substrate and a photosensitive layer made of a suitable diazo compound, are mainly classified into two types, one (negative-type) for using a negative master transparency and the other (positive-type) for using a positive master transparency. These plates normally have the photosensitive layer of nearly the same color, say grayish blue or grayish green, so that the distinction between these two types is hardly expected with the naked eye. In view of this, there have been conventionally employed for automatic development systems a method of identifying the presensitized plates only by detecting the average luminosity or brightness (intensity of black-and-white component) in the reflected light from the photosensitive layer. However, in addition to the printing plates being classified into two basic types, the printing plates in each type are further classified into different types depending on different manufacturers and/or specific purposes, and these plates in each basic type exhibit almost the same color representations so that the distinction by such conventional method between these plates are made much more difficult. For example, spectrum reflectances of several types of commercially available presensitized plates are illustrated in FIG. 1, wherein curves indicated by N-1, N-2, P-1, P-2 and P-3 correspond respectively to the negative-type presensitized plate sold under the tradename of SGN manufactured by Konishiroku Photo Industry Co., Ltd., Japan, the negative-type plate sold under the tradename of GAN manufactured by Fuji Photo Film Co., Ltd., Japan, the positive-type plate sold under the tradename of GAP manufactured by Fuji Photo Film Co., Ltd., the positive-type plate sold under the tradename of SGP manufactured by Konishiroku Photo Industry Co., Ltd. and the positive-type plate sold under the tradename of LKP manufactured by Fuji Pharmaceutical Co., Ltd., Japan. As shown in FIG. 1, the light reflective characteristics of these plates resemble closely with each other, particularly between the plates N-1 and N-2 as well as between the plates P-1 and P-2, from which it is suggested that, even if the distinction between the negative-type and positive-type can be made by the use of the above method which detects an average luminosity of the plate, the distinction among the plates in each type is almost impossible by the same method. Accordingly, an optimum developing procedure suitable for a plate of specific type is hardly to be obtained with this method, which would sometimes lead to undesirable development of the printing plate. This means that such conventional method is no more effective for determining exactly a specific type of or identifying the presensitized plates of different types which have nearly the same light reflective characteristics but require different developing procedures.